Tag Archives: visible light

What is color, and what does it mean for an object to have a specific color? Well, color comes from the fact that light can have different sizes, the way objects reflect that light, and the way our eyes can see it.

Light is made up of these tiny packets of energy, photons, which travel as waves that can move through air or space. And there’s a distance between the peaks of the waves, the same way there would be for waves in water, which is the size of the light. Light can have a whole range of different sizes, so the microwaves that you use to cook food or the radio waves that carry sound through the air are both different sizes of light. But there’s a special range of light, the visible range, which contains the sizes of light that our eyes can detect.

So in the visible range, we have shorter lengths of light, which our eyes see as more blue, and longer lengths of light, which our eyes see as more red. In between, you have the full rainbow, which has all the colors we can see. The sun shines light on us with the whole range of sizes, but different objects will reflect different sizes or colors back at us. So an orange is absorbing most visible light but reflecting orange light, and then our eye detects that light and our brain tells us it’s orange.

But we need special cells in our eyes to detect color. Most people have three kinds of color-detecting cells, called cones, that pick up blue, green, or yellow light. From these three colors, our brain puts together the rest of the rainbow, like an artist does when mixing paint. People who have fewer or more kinds of cones will perceive color differently, maybe being color-blind or seeing even more colors than average, even though the light itself is the same!

I’ve been writing more about light recently, so I wanted to cover a basic question that most people first ask as children: why is the sky blue? We can tell that the blue color of the sky is related to sunlight, because at night, we can see out to the black of space and the stars. We also know it’s related to the atmosphere, because in photos from places like the Moon which have no atmosphere, the sky is black even when the Sun is up. So what’s going on?

When light from the Sun reaches Earth, its photons have a combination of wavelengths (or energies), and we call the sum of all of those the solar spectrum. Some of these wavelengths of light are absorbed by particles in the atmosphere, but others are scattered, which means that the photons in question are deflected to a new direction. Scattering of light happens all around us, because the electromagnetic wave nature of photons makes them very sensitive to variations in the medium through which they travel. Other than absorption of light, scattering is the main phenomenon that affects color.

There are a few different types of scattering. We talked about one type recently, when discussing metamaterials and structural color: light can be scattered by objects that are a size similar to the wavelength of that light. That is called Mie scattering, and it’s why clouds appear solid even though they are mostly empty. The clouds are formed of tiny droplets, around the size of the visible wavelengths of light, and when these droplets scatter white light, the clouds themselves appear diffuse and white. Milk also appears white because it has proteins and fat in tiny droplets, suspended in water, which scatter white light.

However, even objects much smaller than the wavelength of light can induce scattering. The oxygen and nitrogen molecules in the atmosphere can also act as scatterers, in what’s called Rayleigh scattering (or sometimes the Tyndall effect). For Rayleigh scattering, these molecules can be affected by the electromagnetic field that the photon carries. A molecule can be polarized, meaning the positive and negative charges in the molecule move in opposite directions, and then the polarized molecule interacts with the light by scattering it. But, the polarizability of individual molecules depends on the wavelength of the incoming light, meaning that some wavelengths will scatter more strongly than others. When Rayleigh worked out the mathematical form of this dependence, in 1871, he found that the scattering was inversely proportional to the fourth power of the wavelength of light, which means that blue light (which has a smaller wavelength) will scatter much more strongly than red light (which has a larger wavelength).

Thus, we see the Sun as somewhat yellow, because only the longer wavelength light in red and yellow travels directly to us. The shorter wavelength blue light is scattered away into the sky, and comes to our eyes on a very circuitous and scattered route that makes it look like the blue light is coming from the sky itself. At sunset, the sun appears even redder because of the increased amount of atmosphere that the light has travelled through, scattering away even more blue light. And, when there is pollution in the air, the sun can appear redder because there are more scattering centers that scatter away the blue light.

Of course, the fact that blue light scatters more is only half the story. If that were all there is to it, we’d see the sky as a deep violet, because that’s the shortest wavelength of light that our eyes can see. But even though we can see the violet in a rainbow, our eyes are actually much less sensitive to it than they are to blue light. Our eyes perceive color using special neurons called cones, and of the three types of cones, only one can detect blue and violet light. But the blue cone’s response to light peaks at around 450 nm, which is right in the middle of the blue part of the spectrum. So we see the sky as blue because it is the shortest wavelength that we’re capable of detecting in bulk. Different particles in the air can change the color of the sky, but so would different ways of sensing color. So Rayleigh scattering determines which light is scattered, and our visual system determines which of that light we see best: sky blue.

We already know the basics of light: it’s electromagnetic energy, carried through space as a wave, in discrete packets called photons. But photons come in a variety of energies, and different energy photons can be used for different real-world applications. The energy of a photon determines, among other things, how quickly the electromagnetic wave oscillates. Higher energy photons oscillate more quickly than lower energy photons, so we say that high-energy photons have a higher frequency.

This frequency isn’t related to the speed that the photons travel, though. They can oscillate more or fewer times over a given distance, but still traverse that distance in the same amount of time. And as we know, the speed of light is given by Maxwell’s Equations for electromagnetism, and is constant regardless of reference frame! But another way to look at frequency is by considering the wavelength of light. Picture two photons which are traveling through space, at the same speed, but with one oscillating faster than the other. Thus one photon is high-frequency and one is low-frequency. While traversing the same distance, the high-frequency photon will oscillate more times than the low-frequency photon, so the distance covered by each cycle is smaller. We call this distance for a single cycle the wavelength, and it’s inversely proportional to the frequency. Long-wavelength photons are low-frequency, and short-wavelength photons are high frequency. Overall the range of photon frequencies is called the electromagnetic spectrum.

On Earth, photons come from an external source, often the sun, and are reflected off various objects in the world. The photons of a specific color may be absorbed, and thus not seen by an observer, which will make the absorbing object look like the other non-absorbed colors. If there are many absorbed photons or few photons to begin with, an object may just look dark. Our eyes contain molecules capable of detecting photons in the wavelength range 400-700 nanometers and passing that signal to our brains, so this is called the visible wavelength range of the electromagnetic spectrum. But it’s the interaction of photons with the world around us, and then with the sensing apparatus in our eyes, that determines what we see. Other creatures that have sensors for different frequencies of light, or who have more or less than three types of cones, may perceive the color and shape of things to be totally different. And, the visible spectrum is only a small slice of the total range of photon frequencies, as you can see in the image below!

Photons that are slightly lower energy than the visible range are called infrared, and our skin and other warm objects emit photons in the infrared. Night-vision goggles often work using infrared photons, and some kinds of snakes can see infrared. Even lower energy photons have a lot of practical uses: microwave photons can be used to heat material, and radio waves are photons with such low energy that they’re useful for long-range communication! Long wavelength photons are difficult to absorb or alter, so they’re also really useful for astronomy, for example to observe distant planets and stars.

The sun emits photons in the visible range, but it also emits a lot of photons with a slightly higher energy, called ultraviolet or UV. Sunscreen blocks UV frequency photons because they can cause biological tissue to heat up or even burn slightly, and that is sunburn! At even higher frequencies, x-rays are a type of photon that are widely used in biomedical imaging, because they can penetrate tissue and show a basic map of a person’s bones and organs without surgery. And very high energy gamma rays are photons which result from chemical processes in the nuclei of atoms, which can pass through most material. I’ll talk a bit more about x-rays and gamma rays soon, as part of a larger discussion of radiation.

There is a lot more to light than visible light, and the various parts of the electromagnetic spectrum are used in many applications. Each wavelength gives us different information about the world, and we can use technology to extend the view that we’re biologically capable of to include x-rays, infrared, and many other parts of the electromagnetic spectrum!